1. Field of the Invention
The present invention relates to a high-voltage power supply for an image forming apparatus using an electrophotographic method, such as a copying machine and a printer.
2. Description of the Related Art
An electrophotographic image forming apparatus includes, for example, a photosensitive drum as an electrophotographic photosensitive member. In image formation, a charging process is conducted to charge a surface of the photosensitive drum substantially uniformly to a predetermined potential. To conduct the charging process, for example, a contact charging method is used. In the contact charging method, for example, a charging roller as a charging member is brought into contact with the surface of the photosensitive drum, and a voltage is applied to the charging roller to charge the surface of the photosensitive drum.
The contact charging method includes a direct current (DC) charging method. In the DC charging method, a DC voltage value Vd+Vth is applied to the charging roller to charge the surface of the photosensitive drum to a desired potential Vd. The DC voltage value Vd+Vth is a sum of a voltage value Vd, which corresponds to a desired potential Vd of the charging roller, and a voltage value Vth. The voltage value Vth is a discharge start voltage for the photosensitive drum, which is a member to be charged, at the time of application of the DC voltage to the charging roller.
The contact charging method also includes an alternating current (AC) charging method for charging the photosensitive drum more uniformly. In the AC charging method, a charging voltage obtained by superimposition of DC voltage of a voltage value Vd corresponding to the desired potential Vd and AC voltage having a peak-to-peak voltage value (p-p voltage value) equal to or higher than a double of the voltage value Vth is applied to the charging roller. In the AC charging method, the AC voltage is superimposed so that discharges on the positive and negative sides occur alternately to charge the surface of the photosensitive drum more uniformly.
In the AC charging method, if AC voltage of a sine wave is applied to the charging roller, a resistive load current flows in a resistive load between the charging roller and the photosensitive drum, a capacitive load current flows in a capacitive load between the charging roller and the photosensitive drum, and a discharge current flows between the charging roller and the photosensitive drum. In other words, the sum of the currents flows into the charging roller. It is empirically known that it is desirable to set the amount of discharge current to a predetermined amount or larger to stably charge a surface of a photosensitive drum.
However, when an excessive amount of discharge current flows, the photosensitive drum may be scraped to expedite deterioration of the photosensitive drum, or an abnormal image may be formed such as image deletion (distortion of electrostatic latent image due to a decrease in resistance of photosensitive drum) in a hot and humid environment due to a corona product. To prevent such expedition of deterioration of the photosensitive drum and formation of an abnormal image, it is desirable to apply AC voltage to minimize discharges generated to the positive and negative sides alternately.
To stably supply high-quality images over a long period of time, it is required to control the voltage value of AC voltage to be applied to the charging roller and the value of current flowing into the charging roller by application of AC voltage to realize uniform charging without excessive discharging. As a method of such a control, a discharge current control method is discussed in Japanese Patent Application Laid-Open No. 2001-201921 in which the voltage value of AC voltage is determined to obtain a desired amount of discharge current at the time of image formation. In the discharge current control, AC current values are measured at the time of application of AC voltage of a p-p voltage value in a non-discharging range lower than a double of the discharge start voltage Vth and at the time of application of AC voltage of a p-p voltage value in a discharging range equal to or higher than the double of the discharge start voltage Vth. Based on the measurement results, the p-p voltage value of AC voltage at the time of image formation is determined.
To conduct the discharge current control, it is important to output the amplitude of AC voltage with adequate accuracy at each measurement point. In an AC voltage generation circuit, a transformer driving circuit drives a primary side of an AC transformer to generate AC voltage, which is high-voltage, on a second side of the AC transformer. A p-p voltage detection circuit detects the AC voltage on the second side. In the p-p voltage detection circuit, a voltage doubling circuit stores the p-p voltage as DC voltage in a capacitor. Since the voltage is a high voltage of 1 kV or higher, the voltage is divided by two resistors to be converted into a voltage that is applicable to an ordinary operational amplifier integrated circuit (IC). A resistor with a high withstand voltage and a high resistance value (high-voltage resistor) is used as one of the resistors used to divide voltage.
Meanwhile, the photosensitive drum charged to the desired potential Vd is exposed to light such as laser light modulated according to image data. The exposure neutralizes the surface of the photosensitive drum to form a V1 potential, whereby an electrostatic latent image (electrostatic image) is formed on the surface of the photosensitive drum. The electrostatic latent image is developed with toner to form a toner image on the surface of the photosensitive drum. The toner is stored and conveyed by, for example, a development sleeve as a developer bearing member and supplied to the photosensitive drum.
A development DC voltage Vdc is applied to the development sleeve to give an electric potential for development. The image density is determined by a potential difference between the voltage value Vdc and the potential V1. By maintaining the potential difference between the potential Vd and the voltage value Vdc at a predetermined value, carrier contained in a two-component developer and fogging toner (toner adhering to a non-image portion) can be prevented from adhering to the photosensitive drum. Accordingly, adequate accuracy is also required with respect to the voltage values Vd and Vdc.
To generate the voltage values Vd and Vdc, a circuit that divides an output voltage to convert it into a voltage applicable to the operational amplifier IC is used to maintain the voltage values constant. A high-voltage resistor with a high resistance value is used as one of the resistors for use in dividing voltage, as in the p-p voltage detection circuit.
However, the high-voltage power supply for an image forming apparatus that controls an output voltage generated by dividing a high-voltage output by a voltage dividing circuit to be maintained constant, has the following problem to be improved.
In an image forming apparatus, dew condensation may occur due to a change in an installation environment. Especially in the winter morning, when the environmental temperature in a chilly installation environment is increased rapidly due to use of a stove in the installation environment, significant dew condensation may occur. In such a case, dew condensation is likely to occur on a high-voltage power supply circuit board in an image forming apparatus.
Since a high-voltage resistor of a voltage dividing circuit that divides a high-voltage output has a significantly high resistance value, when dew condensation occurs on a circuit board on which the high-voltage resistor is mounted, the dew condensation causes leakage current to decrease a substantial resistance value, and the divided voltage becomes higher than the set value.
In general, a phenolic paper circuit board is used as a printed circuit board for use in a high-voltage power supply. Compared to other glass epoxy circuit boards, a phenolic paper circuit board is less likely to repel water drops formed by dew condensation. As to a high-voltage resistor, if a high-voltage resistor with a resin-coated surface is used, water drops are repelled and a continuous path is less likely to be formed by dew condensation on a surface of the resistor. Since a control circuit controls a divided voltage to be a predetermined value as described above, an output voltage becomes smaller than a desired value.
When an environmental temperature changes, the condition of dew condensation on a circuit board also changes, and a substantial resistance value including leakage current may change every second. If, for example, the discharge current control is performed under the situation in which the substantial resistance value changes, an adequate control result cannot be obtained. Thus, shortage of discharge current may occur during image formation to cause a fogged image (image with toner adhering to a non-image portion) due to charging failure and image deletion due to excessive discharging.
As to the control of the output voltages Vd and Vdc, if dew condensation occurs between terminals of a high-voltage resistor on a circuit board during image formation, a desired voltage may not be output to cause density failure, or an output voltage may change to cause uneven density.
Japanese Patent Application Laid-Open Nos. 2011-204426 and 2011-210603 discuss methods of preventing sparks in a magnetron driving power supply, which may be generated when a high-voltage resistor for discharging a high voltage is short-circuited by water drops adhering to the high-voltage resistor. In the methods, a slit is formed in a printed circuit board under the high-voltage resistor. The methods can reduce a flow of a large amount of current that may lead to a spark. However, the methods cannot adequately prevent generation of minor leakage current.